Electromagnetic shielding

ABSTRACT

Aluminum silicon alloys are used as cores for highly conductive metal coated particles to be incorporated in resins for use as electromagnetic interference shielding. High temperature processing conditions are used after coating of the particles to improve internal and external corrosion resistance of gaskets and filled resins into which the particles are incorporated for shielding use.

BACKGROUND OF THE INVENTION

Electromagnetic interference (EMI) shielding materials are well known inthe art in forms such as gaskets, caulking compounds, adhesives,coatings and the like for a variety of EMI shielding purposes.

In the past, Where high shielding performance is necessary, EMIshielding has tended to use silver particles or silver coated copperparticles dispersed in a resin binder. More recently, aluminum coresilver coated particles as said forth in U.S. Pat. No. 4,507,359 havebeen used to reduce costs while maintaining good electrical and physicalproperties.

Prior art particles have often exhibited disadvantages which include oneor more of, limited high-temperature functioning without loss ofelectrical conductivity, corrosion of particles internally or externallyof gaskets, or in the case of silver coated glass particles, limitedcurrent carrying capacity in cases where electromagnetic pulseconditions are encountered.

Other U.S. patents showing the state of the art include U.S. Pat. Nos.3,140,342; 3,194,860; 3,202,488; 3,476,530; 3,583,930 and 4,434,541; aswell as published Hungarian Pat. Document No. 166,727 (referred to inChemical Abstract No. 159606S).

The present inventions provides an EMI shielding material which avoidshigh cost of pure silver particles while providing good EMI shieldingeffectiveness. The present particles when incorporated in shielding,form end structures which provide desirable features in shielding andare even more effective in corrosion resistance than previous particlesincluding the substantially improved relatively recently developedaluminum core precious metal coated particles of the prior art.Maintaining low resistance or impedance across an EMI joint over thelife of an electronic enclosure is critical for shielding. Corrosion ofthe conductive particles (internal corrosion) or of the flange materialat the joint with a shield such as a gasket (external corrosion) can bea substantial problem. The particles of the invention in addition toincreased corrosion resistance, maintain advantages of such prior artaluminum core particles which include reduction in weight and cost whichare particularly significant in certain applications including areospaceapplications.

BRIEF SUMMARY OF THE DISCLOSURE

It is an object of this invention to provide electrically conductiveparticles for use as a conductive filler in a resin matrix forelectromagnetic shielding use which particles provide high corrosionresistance internally and externally of the resin matrix with desirableshielding properties.

Still another object of this invention is to provide an EMI shieldingmaterial having a volume resistivity effective as an electromagneticshield which has high corrosion resistance and desirable electricallyconductive properties resulting in good shielding properties.

Still another object of this invention is to provide methods of formingelectrically conductive particles for use as conductive fillers in resinmatrices for electromagnetic shielding uses which particles are heattreated by efficient and effective methods to increase corrosionresistance properties of final products.

According to the invention electrically conductive particles for use asa conductive filler in a resin matrix for electromagnetic shielding usecomprise an inner core of an aluminum silicon alloy having from about 5percent to about 20 percent by weight of silicon and an intermediatelayer of metal selected from the group consisting of mercury, platium,copper, chromium, platinum, gold, nickel, tin and zinc or alloys andmixtures thereof with an outer layer of a highly electrically conductivemetal which is preferable a noble metal and more preferably silveralthough other metals such as nickel can be used.

According to a method of this invention electrically conductiveparticles are formed for use as a conductive filler in a resin matrixfor electromagnetic shielding use by heat treating an aluminum alloycore noble metal plated particle having an intermediate metallic layerunder non-oxidizing conditions at high temperature preferably in therange of from 300° C. to 600° C. and preferably in a non-oxidizinggaseous atmosphere.

In an alternate method, the particles are treated at a temperature offrom 300° C. to 600° C. at a vacuum condition of 1 Torr or lesspressure.

The metal particles having the silver or other noble metal outer layerand produced in accordance with the invention can be incorporated inresin matrices as for example set forth in U.S. Pat. No. 4,507,359. Theparticles can be of any desired shape known in the art for fillerparticles to make electrically conductive materials. Thus, the particlescan be substantially spherical, flat, non-spherical in the shape of rodsor non-regular geometric shapes. The "resin" as used herein into whichthe particles can be incorporated can be any plastic or elasticmaterials including rubbers such as silicone, flurosilicone,polyisobutylene elastomers. Other plastics suitable for use as the resininclude polyamides, acrylics, urethanes, polyvinyl chloride, siliconeand others as conventionally used in gaskets, adhesives, caulkingcompounds and coatings into which the particles of this invention can beincorporated in known EMI shielding configurations and structures.

It is a feature of this invention that good electrically conductiveproperties and thus high shielding ability can be obtained in economicalproducts capable of good production rates at reasonable costs.Outstanding internal and external corrosion resistant properties areobtained when the heat treating steps of this invention are usedalthough substantially reasonable results are obtained with theparticles even without the additional processing of the high temperaturesteps.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of a compressible gasket according to thisinvention between two wave guide flanges;

FIG. 2 is a front view of the gasket;

FIG. 3 is a sectional view of FIG. 2 taken along line 3--3;

FIG. 4 is a front view of an O ring gasket;

FIG. 5 is a sectional view of FIG. 4 taken along line 5--5;

FIG. 6 is a sectional view of an extruded channel strip gasket;

FIG. 7 is a diagrammatic drawing of steps in initially processingconductive particles in accordance with the present invention;

FIG. 8 is a diagrammatic showing of additional steps for heat treatmentin accordance with the most preferred embodiments of this invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference should now be had to FIGS. 1 to 3. In these FIGS. a die-cutform stable gasket 20 is placed between the flanges 21⁻¹ and 22⁻¹ ofwave-guides 21 and 22 which are held together by bolts 23. The gasket 20being of a compressible binder effectively seals the junction againstmoisture entering the wave-guides as well as preventing electromagneticenergy in the wave-guide from escaping as is known in the art. While thegasket may be cut from a sheet of material it should be appreciated thatit may also be molded. The gasket 20 has a central opening for theelectromagnetic energy to pass from one wave-guide section to anotherand holes 20-2 for passage of bolts 23.

In FIGS. 4 and 5 there is disclosed a molded O ring 30 as anelectromagnetic energy shield. The O ring 30 is placed in grooves of twoflanges such as 21⁻¹ and 22⁻¹ and pressure is applied to compress the Oring to achieve an effective seal.

FIG. 6 illustrates an extruded channel strip gasket 31 made of theshielding material disclosed herein.

It is clear from the above that gaskets (die-cut, extruded, molded) ofmany shapes can be made using the material of this disclosure andaccordingly the invention herein should not be considered as limited toany particular shape.

In the present invention, the most preferred material for use as aconductive particle loaded gasket material comprises a silicone rubberbinder filled with electrically conductive particles having an aluminumsilicon core with a first coating of zinc having an overcoating ofsilver.

Other binders or resins which form the matrix material can includeflurosilicone rubber and others mentioned previously in thespecification. The material can be formed as sheets, molded articles,coatings, adhesives in all known structural forms depending upon thenature of the binder used and the specific application.

Conventional processing steps can be used to incorporate the metalparticles into the resins along with conventional fillers known in theart.

While a zinc intermediate layer is preferred for use with the aluminumsilicon core, other intermediate layers including the following:mercury, palladium, copper, chromium, platinum, gold, nickel, and tin,which can then be coated with a metal and preferably silver can be used.

EMI shielding material preferably comprises 80 to 50 percent by volumeby resin or binder and 20 to 50 percent by volume of electricallyconductive particles of this invention. The volume resistivity of theshielding material is preferably less than 0.1 and most preferably lessthan 0.01 ohm centimeter. Preferably the particle loading in the resinis from 30 to 42 percent by volume and even more preferably from 35 to40 percent by volume. The particles preferably are made up of 1 to 5percent by weight of the intermediate layer although they can have from0.2 to 10 percent by weight of the intermediate layer with preferablyfrom 4 to 25 percent by weight of the overall particle, of the outerlayer which can be silver.

The particles when coated and completed can be in sizes as customarilyin the art for EMI shielding purposes. Preferably the particles have 10to 150 microns in average particles size and most preferably 20 to 75microns in average particles size. The particles can have averageparticle sizes of from 1 to 300 microns or higher.

The silicon aluminum alloy preferably has from about 70 to about 95percent by weight of aluminum and from about 5 to about 20 percent byweight of silicon with from about 9 to 14 percent by weight of siliconbeing preferred. Impurities and small or trace amounts of other metalscomprising conventional aluminum alloying materials may be present inamounts up to about 10 percent by weight as known in the art.

The aluminum silicon alloy powder can be any aluminum silicon powder butis preferably an eutectic alloy. Powders such as commercial aluminumsilicone eutectic alloys available from Alcoa, (Pittsburgh, Pa.) asgrade 718, Reynolds Aluminum, (Richmond, Va.) as LS-A-887 and Valimet(Stockton, Calif.) (coarse and fine grade aluminum silicon powder) canbe used. Such powders customarily range from 5 to 300 microns inparticle size, with often from 5 to 15 percent silicon and a maximum of5 percent of other elements.

It is preferred to use aluminum silicon powders which are atomizedpowders produced with inert gases to insure minimum oxide formation onthe powder surface. As atomized, aluminum silicon powder of eutectic ornear eutectic composition preferred for use in this invention, generallycontain very fine dendritic and silicon rich percipitates throughouteach particle. The morphology of the dentrites can be coarsened andspheroidized by heat treating the powder above 300° C. and preferably inthe range of 300° C. to 600° C. When the structures are examined under amicroscope using samples prepared by conventional metallographicpolishing and etching techniques the dendritic structure can be seen.

In the process of treating the aluminum silicon powder, as shown in FIG.7, in a first activating step the powder is etched using a mild alkalinesolution to create a silicon rich surface. Etching can be carried outwith acid or base or in water (with heat) preferably at a ph of under10. In each case the solution is exothermic even though no heat need beused with the acid or base treatment. The temperature is controlled tomaintain the processed powders so as to prevent runaway reactions whichcould become violent; yet, allow removal of naturally formed oxidelayers, in time periods of from 2 to 15 minutes or longer.

In a second step as illustrated in FIG. 7, a rinse with water or anywashing solution is used without allowing the particles to dry.

Plating can then be carried out with the intermediate layer using knowntechniques as known in the art and as described in U.S. Pat. No.4,507,359.

Standard formulae for treatment of bulk aluminum parts may be found inthe 1978 Metal Finishing Guidebook Directory, 46th Annual Edition, pp171-172, published by Metals and Plastics Publications, Inc. Hackensack,NJ, as follows:

    ______________________________________                                        1.     Sodium Hydroxide                                                                             70     oz/gal                                                                              524    .sub.g /l                                  Zinc Oxide     13     oz/gal                                                                              97     .sub.g /l                           2.     Sodium Hydroxide                                                                             6.7    oz/gal                                                                              50     .sub.g /l                                  Zinc Oxide     0.67   oz/gal                                                                              5      .sub.g /l                           3.     Sodium Hydroxide                                                                             16     oz/gal                                                                              120    .sub.g /l                                  Zinc Oxide     2.7    oz/gal                                                                              20     .sub.g /l                           ______________________________________                                    

Using aluminum silicon powder, the amount of sodium hydroxide presentshould be controlled since its reaction with aluminum silicon powder canbe violent and exothermic. In order to keep the reaction under controlwith relatively fine aluminum silicon powders (ca 60 micron averageparticle size) one should preferably use a solution containingapproximately 10 grams of sodium hydroxide and 1.5 grams of zinc oxidewith 100 grams of aluminum silicon powder (Valimet (coarse grade)) inabout 750 mil deionized water. Good results are obtained when thereaction mixture is stirred for one hour. The powder is allowed tosettle and is rinsed five times. The sodium hydroxide zinc oxidetreatment is then repeated and the powder rinsed five times. It has beenfound that a second zincate treatment is preferred with powder toachieve optimum properties after silver plating.

In order to sensitize the powder for silver plating, the zinc coatedaluminum silicon powder is immersed in a dilute solution of a reducingagent. In theory, the reducing agent is absorbed on the powder surfaceand initiates silver plating at the adsorption sites. In practice a 100gram sample of zincated powder is dispersed in approximately 750 ml ofdeionized water containing 37 percent formaldehyde and stirred fifteenminutes allowed to settle and is rinsed three or four times.

Silver plating is carried out by conventional methods. The sensitizedpowder is dispersed in a solution prepared by dissolving 30 grams ofsilver nitrate in 500 ml deionized water and adding approximately 50 mlof 28 percent ammonium hydroxide. To this dispersion, approximately 150ml of 37 percent formaldehyde is added over 15 minutes. The silverplated powder, which is a light tan color, is washed and rinsed severaltimes with water and then washed with acetone and oven dried. After thepowder is dry, it is heat treated at 200° C. at a pressure of 1 Torr forthree hours prior to use. Aluminum silicon powder coated with zinc andthen silver plated in this manner is highly conductive.

Another metal which may be used to form a composite particle withaluminum silicon and silver is nickel. The aluminum silicon powder isimmersion coated with nickel from an acid solution preferably containingchloride or fluoride ions which aid in removal of the oxide coating fromthe aluminum. The aforementioned 1978 Metal Finishing Guidebook andDirectory (p. 484) describes an immersion process for deposition ofnickel on aluminum using 11 grams per liter of nickel sulfate and 30grams per liter of ammonium chloride used at a boil. It is found thatdoubling the nickel concentration gives improved silver platingcharacteristics to the aluminum silicon powder. Thus, in a typicalexperiment, 100 g of aluminum silicon powder is dispersed in 750 ml ofdeionized water containing 20 grams of nickel sulfate and 30 grams ofammonium chloride. The dispersion is heated to about 95° C. and stirredfor one hour. The powder is allowed to settle and rinsed five times.

Sensitization for silver plating is carried out using methodsconventional for nonconductors. First, the powder is dispersed in asolution containing 1 gram per liter stannous chloride and 4 grams perliter of 36 percent hydrochloric acid.

After five rinses, the powder was dispersed in a solution containing 0.2g/l palladium chloride and 0.2 g/l 36 percent hydrochloric acid. Afterstirring fifteen minutes, the powder is allowed to settle and rinsedfive times.

Plating is carries out by dispersing the powder in a solution preparedby dissolving 30 grams of silver nitrate in 500 ml deionized water andadding approximately 50 ml of 28 percent ammonium hydroxide. To thisdispersion about 150 ml of 37 percent formaldehyde is added over 15minutes. The powder is washed several times with water, rinsed withacetone and dried. The dry powder is heat treated for three hours at200° C. at 10⁻² Torr.

Tin can be displacement plated on aluminum silicon powder e.g., Valimet(coarse grade) using alkaline solutions of tin compounds. Theaforementioned 1978 Metal Finishing Guidebook and Directory, page 484discloses 6 oz/gal (45 g/l) sodium stannate at 125°-180° F. for platingtin on aluminum. With aluminum silicon powder lower concentrations ofsodium stannate and lower temperatures should be used because of theextreme exothermic reaction. In practice 100 g of aluminum siliconpowder is dispersed in 700 ml of water and a solution of 13 grams ofsodium stannate is added over 30 minutes.

The mixture is stirred for one hour and allowed to settle and is rinsedfive times. The stannate treatment is repeated and the powder is dried.Sensitizing and silver plating are carried out as with zinc.

It should be understood that other known methods may also be used toeffect plating of aluminum silicon.

Note FIG. 7 shows the intermediate layer coating as stannating or zinccoating with silver plating as discussed.

After silver plated the particles are rinsed with water or other rinsingsolutions, dried and sifted in accordance with known techniques.

The silver plated aluminum silicon core materials are preferably vacuumheat treated for from 2 to about 8 hours preferably at temperatures offrom 150° C. to 300° C. and preferably about 200° C. and at pressuresless than 1 Torr and preferably of from 10⁻¹ to 10⁻² Torr. This step isa drying step and the temperature, times and vacuum conditions can varygreatly depending on what may be desired to obtain clean dry particleswhich can be incorporated in resin for shielding use, or first heattreated in accordance with a method of FIG. 8.

The particles or other metal coated particles made by conventionalmethods for shielding can then be put in gasket, adhesive or otherapplication materials in accordance with known techniques and are foundto have substantial shielding internal and external corrosion resistanceand other desirable properties. However, more desirable properties withrespect to external corrosion resistance can be obtained by heattreatment of the particles in accordance with the diagrammatic showingof FIG. 8.

As best shown in FIG. 8, an improved corrosion resistant property can beobtained by either of two methods. In a first heat treating method, thepreviously treated particles or other conductive particles having pluralmetallic layers and made by conventional methods, can be treated at hightemperature in a non-oxidizing atmosphere such a nitrogen or argonatmosphere at a temperature of at least 300° C. to 600° C. for a periodof time sufficient to reach the desired temperature up to several hoursand preferably two hours. 530° C. is a preferred temperature whichallows complete phase transformation of the aluminum silicon materialcausing the spheriodization of originally dendritic precipitates in thecore material. Longer time periods can be used although they are notfound to make any substantial difference.

In a second heat treating method as diagrammatically shown in FIG. 8,the same temperature range, i.e., 300° C. to 600° C. can be used forsubstantially the same time period but at a high vacuum rather than agaseous non-oxidizing atmosphere a vacuum of less than 1 Torr as forexample 10⁻² Torr can be used. Thus shielding materials of thisinvention can employ particles treated in accordance with FIG. 7 orfurther processed in accordance with either method of FIG. 8.Additionally either of the methods of FIG. 8 can be used to heat treatconventional aluminum core conductive metal coated particles where thealuminum core is not an aluminum silicon alloy as for example theparticles specifically described in the examples of U.S. Pat. No.4,507,359.

After the heat treating corrosion resistance steps of FIG. 8 or afterthe processing of FIG. 7, as desired, the coated particles can becompounded with resins in accordance with this invention to form variousstructural forms for shielding as known in the art.

The silver plating step of this invention can be any conventional silverplating step including electroplating and electroless silver plating.

The processes of FIG. 8 are believed to act to refine the silver layercausing recrystalization and grain encoarsing to give better largergrain size and better conductivity. In addition the refinement, thealuminum silicon core changes from a dendritic silicon rich precipitateto a more spherical shape with an annealed or refined characteristicwhich can give better conductivity. Moreover, the heat treating steps ofthis invention can act to change the silver substrate interface to aidin conductivity and produce improved corrosion resistancecharacteristics.

The silver or other metallic plating of the aluminum silicon core withits overlying intermediate coating can be carried out by standardmethods as previously described. In addition to the above describedspecific step, aluminum silicon powder can be etched with highconcentrations of sodium hydroxide as for example 600 grams of sodiumhydroxide in 84 liters of water for 14 kilogram batch. The etch processpreferably has two or three rinses between etch steps. A significantamount of aluminum can be removed from the core leaving a silicon richsurface. The etching is important for preprocessing before applying theintermediate layers such as zinc or the other materials of thisinvention to achieve optimum adhesion of the intermediate layer and toallow controllable chemical reaction in the process.

The intermediate layer when it be tin or zinc can be processed at 50° C.and in an alkaline solution. More than one processing step with zinc ortin can be used. For example the powder can be rinsed three timesbetween separate coating or metalizing steps. The process can use 1,040grams of sodium stannate for each stannate step for a 13 kilogram batch.An additional 104 grams of potassium sodium tartrate can be used as abuffer complexer. The sodium stannate and buffer are dissolved in waterand added over 15 minutes. In zincating solutions containing 336 gramsof sodium hydroxide, 84 grams of zinc oxide and 8.4 grams of potassiumtartrate with 0.84 grams ferrichloride added over 15 minutes for eachzincate step.

In stannating, a considerable amount of free tin is produced in theprocess. Most of the tin is large particle size and is removed bysifting the final product. Analysis of stannated powder shows 0.3 to 0.7percent tin. Aluminum silicon powder has been stannated to yield aproduct which is readily silver plated. Zinc levels between 0.5 and 1percent have been easily obtained.

Zincated aluminum silicon powder is sensitized by treatment with asolution of sodium thiosulfate and a small amount of amoniated silvernitrate and then silver plated by adding amoniated silver nitratefollowed by addition of formaldehyde at 300 ml per minute.

The optimum silver content for a powder with an average particle size ofabout 60 microns is 16 percent, but with zincated aluminum silicone itis possible to plate at 8 percent silver and obtain a product whichshows only slightly increased electrical resistance in a gasket.

The following non limiting examples show particluar preferredembodiments of the present invention.

EXAMPLES EXAMPLE I

A highly electrically conductive sheet from which the die-cut gasket ofFIGS. 1 to 3 is made, or produced is described below. Thirty-three andone-half (33.5) grams of a conventional Dow Corning (Midland, Mich.)silicone gum (resin) e.g. #440 is mill mixed with 3.76 grams ofCAB-O-SIL MS7 silica, 0.29 grams of R. T. Vanderbilt Varox(2,5-dimethyl, 2,5-di (t-butylperoxy) hexane. To this mixture on themill is added 62 grams of particles of aluminum silicon having anintermediate layer of zinc and an outer layer of silver, (powder) 60microns average size and mixing is continued to homogenuity. Thealuminum silicon core particles are of an eutectic alloy of aluminum andsilicon and are made as illustrated in FIG. 7 and then heat treated at530° C. for two hours in a nitrogen atmosphere. The resin particlemixture is sheeted off the mill 62 mils thick and is placed in a moldand molded at 325° F. at 30 ton pressure for 15 minutes. After removalfrom the mold, the material is post cured at 350° F. for three hours.The volume percent is 37.5 volume percent particles with the gum (resin)being 49.3 volume percent. The gasket is then die-cut from the sheet.Typical volume resistivity of such gasket is 0.002-0.005 ohm-cm.

EXAMPLE II

An electrically conductive adhesive is prepared by mixing 75 parts byweight of silver - zinc - aluminum silicon powder (60 microns averageparticle size) as described above in a solution of 20 parts by weight ofsolid polyamide resin (Versalon 1100) 5 parts by weight of liquidpolyamide resin (Versamide 125) 25 parts by weight of toluene 25 partsby weight of ethanol.

EXAMPLE III

An electrically conductive caulking is prepared by mixing 288 parts byweight of silver - zinc - aluminum silicon powder (60 microns averageparticle size) as described above to a solution of

34 parts by weight of toluene

34 parts by weight of ethanol

32 parts by weight of polyamide resin

EXAMPLE IV

When the silver outer layer zinc intermediate layer aluminum siliconcore particles are replaced in any of Examples I-III with particleshaving intermediate layers of tin or nickel advantageous corrosionresistant properties are obtained.

EXAMPLE V

In a corrosion test the gasket material made as in Example I is die-cutand flanged in between aluminum plate. The fixture gasket is then testedfollowing a procedure described as ASTM B117-73 salt fog testing. After144 hours the test fixture is then washed with warm water. Minimalcorrosion is observed showing desirable external corrosioncharacteristics as compared with conventional gaskets having silver,silver on glass and silver on copper particles. The gasket material doesnot lose its electrical properties and remains effective as an EMIshield.

I claim:
 1. An electrically conductive particle for use as a conductivefiller in a resin matrix suitable for electromagnetic shielding use,saidparticle comprising an inner core of an aluminum silicon alloy havingfrom 5 to 20 percent by weight of silicon, an intermediate layer of ametal selected from the group consisting of mercury, palladium, copper,chromium, platinum, gold, nickel, tin, zinc and mixtures thereof, and anouter layer of a highly electrically conductive metal.
 2. A particle inaccordance with claim 1 and further comprising said aluminum siliconalloy being an eutectic alloy.
 3. An electrically conductive particle inaccordance with claim 1 and further comprising said last mentionedhighly electrically conductive metal being silver in an amount of from 4percent to 25 percent by weight of said particle.
 4. An electricallyconductive particle in accordance with claim 1 wherein said intermediatelayer comprises from 0.1 percent to 5 percent by weight of saidparticle.
 5. An electrically conductive particle in accordance withclaim 4 wherein said intermediate layer is zinc.
 6. An electricallyconductive particle in accordance with claim 3 wherein said intermediatelayer is tin.
 7. An electrically conductive particle in accordance withclaim 1 wherein said intermediate layer is zinc.
 8. An electromagneticenergy shielding material having a volume resistivity to be effective asan electromagnetic energy shield, said material comprising resin matrixloaded with electrically conductive particles, said particles comprisingan aluminum silicon alloy core having a first layer of a materialselected from the group consisting of mercury, palladium, copper,chromium, platinum, gold, nickel, tin and zinc,and an outer layer of ahighly electrically conductive metal overlying said first layer.
 9. Theshielding material in accordance with claim 8 whereas said particleshave an intermediate layer of a metal selected from the group consistingof mercury, palatium, copper, chromium, platinum, gold, nickel, tin,zinc and mixtures thereof.
 10. Shielding material in accordance withclaim 9 whereas said intermediate layer is zinc.
 11. A shieldingmaterial in accordance with claim 9 whereas said intermediate layer istin.
 12. A shielding material in accordance with claim 9 whereas saidintermedite layer is nickel.
 13. A shielding material in accordance withclaim 8 whereas said outer layer is silver.
 14. A shielding material inaccordance with claim 13 wherein said outer layer is silver in an amountof from about 4 percent to about 25 percent by weight of said particle.15. A shielding material in accordance with claim 8 wherein saidaluminum silicon alloy has from about 70 to about 95 percent by weightof aluminum and from about 5 to about 20 percent by weight of silicon.16. A shielding material in accordance with claim 15 wherein saidsilicon is present in an amount of from 9 to 14 percent and said alloyis an eutectic alloy.
 17. A shielding material in accordance with claim16 comprising from about 80 to about 50 percent by volume of resin andfrom about 20 to about 50 percent by volume of said particles.
 18. Ashielding material in accordance with claim 17 wherein said particleshave an average particle size of from about 20 to 75 microns,saidintermediate layer is present in an amount of from 0.2 to about 10percent by weight of said particle and said electrically conductiveouter layer is present in an amount of from about 4 to about 25 percentby weight of said particle.